Guardrails have been used for many years on our nation's highways to protected errant motorists from hazards alongside the roadway. Guardrails function by capturing errant vehicles and redirecting them away from the hazard. Hazards that are commonly protected by guardrails include trees, signs, culverts, bridge piers, steep edge drop-offs, and soft soil that could cause a vehicle to roll.
Guardrails are able to capture and redirect an errant vehicle because they have the longitudinal strength to resist the vehicle impact. This means that the steel rail and its joints are stronger than the forces generated during the vehicle impact. The steel rail is held in place by either wood or steel posts. The posts hold the rail at the proper height and are designed to bend over and fail during an impact. These posts are individually relatively weak, however when taken as a system, they are able to resist the lateral loads imposed upon the rail. Additional structural strength is provided to the rail by anchoring each end of the rail, either through the use of a crashworthy end terminal, or some other means of fixing the end of the steel rail to the ground.
Traditional guardrail systems, such as disclosed by U.S. Pat. No. 3,493,213 to Ackerman, consist of a rail which is attached to a supporting post via an offset bracket or “block-out”. The offset brackets hold the guard rail panel away from its supporting posts so as to help prevent snagging of an impacting vehicle's wheels on the posts. Various types of offset brackets are commercially available, including wood blocks (hence the term “block-out”), steel I-beam sections, and also blocks formed of elastomeric materials, such as is disclosed by U.S. Pat. No. 6,530,560 to King.
Block-outs also may help maintain the height of the guardrail during a vehicle impact. For example, when a vehicle impacts a guardrail system with blockouts, the vehicle imparts lateral forces onto the rail. These forces are transmitted to the block-outs, which then transmit them to the support posts. The support posts may tend to rotate during the impact. Since the guardrail and blockouts are attached to the posts they also rotate on an arc generally centered at the point where the post is embedded in the soil. If the guardrail were directly connected to the post, this rotation would result in the guardrail being pulled downward, closer to the ground. But since the guardrail is spaced from the post, the rotation initially results in a slight gain in height of the guardrail, rather than a loss of height. Maintaining the guardrail at a consistent height may help prevent an impacting vehicle from riding up over the guardrail.
U.S. Pat. No. 7,530,548 to Ochoa discloses a guardrail system where the guardrail is directly connected to the post via a releasable fastener. The Ochoa system prevents issues with wheel snag and the guardrail being pulled down by an impact by using a weak fastener to hold the rail to the post. Because of this, the rail is released from the post very soon after a vehicle impact. This prevents the rail and the post working together to snag the wheels of an impacting vehicle. The released rail also cannot be pulled downwards by the post as it rotates during the impact.
U.S. Pat. No. 7,878,485 to Conway discloses a guardrail system that uses a standard guardrail bolt, with a washer between the post and the guardrail. A slot allows the rail to remain at generally the same height, without disengaging from the post, as the post rotates and moves laterally during a vehicle impact. Because the post continues to hold the guardrail during much of the impact event, the post continues to restrain the rail and resists additional lateral movement.
In a similar manner, U.S. Patent Application 20120003039 to Wallace discloses a guardrail system that consists of a carriage that attaches the guardrail to the support post. When the system is impacted, the carriage is free to move upwards, but is prevented from moving downwards by an indentation in the post. Although both the Wallace and Conway systems retain the guardrail, while preventing the rotation of the post from pulling it downwards, the systems do not capture and retain the guardrail at an appropriate pre-impact height, nor do they have a means of limiting the movement of the rail up the post. For instance, guardrail systems are subject to a variety of nuisance impacts which may flex the guardrail system, without permanently deforming it or causing significant damage such as low speed impacts by vehicles, bicycles, pedestrians, or wildlife. The guardrail may also be subject to various environmental forces, such as high winds, temperature fluctuations, and high snowfall. The effects of temperature fluctuations and snowfall may combine to create particularly harsh conditions for the guardrail. Temperature fluctuations may cause the fasteners in a guardrail system to loosen over time and this is particularly troublesome for guardrail system such as the Wallace and Conway designs that depend upon the tightness of fasteners to properly locate the rail. Once the fasteners in these designs are loosened, the rail is subject to misalignment from the nuisance impacts listed previously, any also from the effects of snowfall, and the forces transmitted to the rail by passing snowplows during its removal.